The sidekick will shoot well with pellets I am sure. Their inline design and the 209 ignition will create a positive ignition. As for sidelocks, the T/C Firestorm claims to shoot pellets even as a flintlock, but I believe they advise you to duplex the load with a little loose black powder down by the vent hole first. Also the Tradition's PA Pellet will also shoot pellets as a flintlock. Again, a duplex is probably a good idea. I never monkeyed with them so I can not say.

Don't try shooting the FFFFg out of a rifle. That is mainly for pan flash. It is far to fine to control as a main charge. Yes, many people shoot FFFg out of .54 calibers. I do. I also shoot Triple Se7en out of them. They will shoot about anything. You just have to try them out and see what works. As for absorbing moisture, all powders will. Some are worse then others. So far of all the powders I have "played with on the range" but not in a hunting situation have not gave me moisture problems. Actually I have never had a problem with any powder in hunting situations yet.

There are others here that can give you a much more details list of what powders are better or worse when it comes to powder or moisture.